Implantable pacing devices often include electrodes to deliver pulses and/or to sense cardiac activity in response to delivered pulses. Cardiac activity related to a delivered pulse is typically known as an “evoked response” (e.g., an electrical signal arising from atrial or ventricular cardiac tissue depolarization in response to delivery of a pacing pulse). However, many issues complicate sensing and/or detection of evoked responses. For example, post-pulse electrode polarization can interfere with detection of an evoked response and/or produce a “polarization artifact” in a detected signal. Post-pulse electrode polarization results primarily from capacitive charging of an electrode-electrolyte interface during delivery of a pacing pulse. Upon termination of the pacing pulse, the post-pulse electrode polarization decays over time, generally in an exponential fashion like a capacitor. Characteristics of post-pulse electrode polarization generally depend on a variety of parameters, such as, electrode materials, electrode geometry, tissue characteristics, tissue contact, stimulation energy, and others, many of which vary over time. Consequently, an elaborate characterization of post-pulse electrode polarization is impractical, especially when one considers resource limitations inherent in implantable pacing devices.
U.S. Pat. No. 6,163,724, entitled “Microprocessor capture detection circuit and method,” to Hemming et al. ('724 patent), addresses post-pulse electrode polarization through use of filtering. More specifically, the '724 patent discloses “an adaptive nonlinear filtering technique referred to as ‘Negative Peak Tracking’ (or ‘NPT’) that removes the initial residual [post-pulse] polarization signal, and then passes only that portion of the sensed signal where a change in the sign of the slope occurs” (col. 7, lines 4-8). According to the '724 patent, such filtering, in combination with “[j]udicious selection of comparator threshold levels by a user[,] improves the reliability of event discrimination [capture versus non-capture events]” (col. 7, lines 26-28).
Another issue in detection of evoked responses stems from differences in ventricular and atrial pacing. Thus, approaches to detection of evoked responses in ventricular pacing may not apply directly to detection of evoked responses in atrial pacing. For example, U.S. Pat. No. 5,713,934, entitled “Evoked and spontaneous cardiac activity detection in a dual-chamber electronic pacemaker and method,” to Leckrone, ('934 patent) discloses a pacing system that uses ventricular pacing electrodes and atrial pacing electrodes for sensing. According to the '934 patent, a ventricular evoked response typically peaks at about 30 milliseconds following a ventricular pulse and ranges in amplitude from about 3 millivolts to about 20 millivolts; whereas, an atrial evoked response typically occurs within 20 milliseconds of an atrial pulse with an amplitude generally smaller than that of a ventricular evoked response.
While other pacing and sensing electrode configurations are possible, none of the patents referred to herein discuss the relationship between post-pulse timing of an atrial evoked response and electrode configuration. For example, U.S. Pat. No. 4,549,548, entitled “Pacemaker system with automatic event-programmed switching between unipolar and bipolar operation,” to Wittkampf et al., issued Oct. 29, 1985 ('548 patent), discloses a pacemaker system capable of both unipolar and bipolar sensing wherein “considerations for unipolar and bipolar sensing vary at different times in the pacing cycle, dependent upon the next anticipated event” (col. 2, II. 12-19). Thus, the '548 patent focuses on the relationship between electrode configuration and “the next anticipated event” and not other considerations such as, but not limited to, electrode polarization. Likewise, U.S. Pat. No. 4,858,610, entitled “Detection of Cardiac Evoked Potentials,” to Callaghan et al., issued Aug. 22, 1989 ('610 patent), fails to discuss the relationship between post-pulse timing of an atrial evoked response and electrode configuration.
U.S. Pat. No. 5,873,898, entitled “Capture Detection Circuit for Pulses and Physiological Signals,” to Hemming et al., issued Feb. 23, 1999 ('898 patent), discloses a system for pacing and sensing and presents data from a canine ventricular pacing study for pacing and sensing in both unipolar and bipolar electrode configurations. More specifically, in Table 3 of the '898 patent, “Event 15” uses a ring-to-can electrode configuration for ventricular sensing (col. 25, II. 56-57). While the '898 patent alludes to enhanced accuracy for tip-to-can (unipolar) ventricular sensing when compared to tip-to-ring (bipolar) ventricular sensing (col. 29, II. 51-55), the '898 patent makes no further mention of “Event 15”. As shown in Table 4 of the '898 patent, for Event 15, the capture detection circuit had a success rate index of less than one for three of the six pacing settings tested (col. 28, II. 36-40). Therefore, the '898 patent suggests that ring-to-can sensing is not beneficial for detection of ventricular evoked responses.
None of the aforementioned patents discuss an atrial pacing and sensing system that diminishes and/or eliminates the effects of electrode polarization through electrode configuration. In particular, none of the aforementioned patents disclose an atrial pacing and sensing system wherein detection of an atrial evoked response occurs more than approximately 20 milliseconds after administration of an atrial pulse and/or wherein an atrial evoked response is relatively independent of pulse power.